1. Field of the Invention
The present invention relates to a power supply circuit and, more particularly, to a switching power supply circuit which operates with a wide range of input DC voltage and is economical.
2. Description of the Related Art
FIG. 4 shows a switching power supply circuit (F-Z power supply) for DC operation incorporating a standby power supply in a power supply system of a conventional fixed switching frequency and series resonance frequency control type. In this power supply circuit, an input DC voltage E1 of 10.5 to 24V from a battery or the like is applied to the collectors of switching transistors Q.sub.1 and Q.sub.3 of first and third half-bridge resonant converters, respectively. The first half-bridge resonant converter is formed of the transistor Q.sub.1, a capacitor C.sub.B1, a portion of the primary winding of a converter drive transformer CDT, etc., while a second half-bridge resonant converter is formed of a transistor Q.sub.2, a capacitor C.sub.B2, a portion of the primary winding of the converter drive transformer CDT, etc.
Further, the third half-bridge resonant converter is formed of the transistor Q.sub.3, a capacitor C.sub.B3, a portion of the secondary winding of the transformer CDT, etc., while a fourth half-bridge resonant converter is formed of a transistor Q.sub.4, a capacitor C.sub.B4, a portion of the secondary winding of the transformer CDT, etc.
In this case, larger collector currents flow through the transistors Q.sub.1 to Q.sub.4 the lower the input DC voltage is. Therefore, those of high current amplification are selected. Further, in order to reduce the drive currents of the switching transistors Q.sub.1 to Q.sub.4, starting currents are supplied thereto through starting resistors R.sub.S1 to R.sub.S4 as described later, and accordingly, the DC current flowing into the collector at this time becomes Ic=h.sub.FE .multidot.I.sub.B (where h.sub.FE =200 to 300).
The emitter of the transistor Q.sub.1 and the collector of the transistor Q.sub.2 are connected to one end of the secondary winding N.sub.1 ' of a saturable power regulation transformer PRT through a capacitor C.sub.1, and the emitter of the transistor Q.sub.3 and the collector of the transistor Q.sub.4 are connected to the other end of the secondary winding N.sub.1 ' of the transformer PRT through a relay contact ry1 of a two-circuit one-contact electromagnetic relay RY. From the secondary winding N.sub.3 of the transformer PRT, a DC voltage of 15V is taken out through diodes D.sub.1 and D.sub.4 and a DC voltage of 7.5V is taken out through diodes D.sub.2 and D.sub.3. From the secondary winding N.sub.2 of the transformer PRT, a DC voltage E.sub.0 of 115V is taken out through a bridge type rectifier D. This DC voltage E.sub.0 is also applied to the control winding Nc of the transformer PRT.
In order to reduce the driving power, the bases of the switching transistor Q.sub.1 and Q.sub.3 of the first and third half-bridge resonant converters are supplied with a DC voltage of 12V through a relay contact ry2 of the two-circuit one-contact electromagnetic relay RY and the starting resistors R.sub.S1 and R.sub.S3, while the emitters of the switching transistors Q.sub.1 and Q.sub.3 are connected with the bases of the second and fourth transistors Q.sub.2 and Q.sub.4 through the starting resistors R.sub.S2 and R.sub.S4, respectively. Here, a current of 5V/50 mA is supplied to a remote control receiver, not shown, and a transistor Q.sub.5 is turned on by an on signal of the main power supply from the remote control receiver. Thereby, the electromagnetic relay RY is driven and the DC voltage E1 of 12V is supplied to the bases of the switching transistors Q.sub.1 and Q.sub.3 through the relay contact ry2 and starting resistors R.sub.S1 and R.sub.S3.
However, since the above described conventional power supply circuit is formed with current resonant converters of a fixed switching frequency type, the range within which stabilization of the DC voltage E.sub.0 is secured is the range of the DC voltage E1 from 10.5 to 24V. When a battery used is that of a rated voltage of 24V as is the case with a bus or a ship, the battery voltage varies over the range of 24.+-.8V. Therefore, in the range of the input DC voltage from 24 to 32V, there arises a difficulty that stabilization of the DC voltage E.sub.0 cannot be secured as indicated in FIG. 3 by the broken line.
Further, at the time when the circuit is in the standby state and the electromagnetic relay RY is off, small currents flow from the transistor Q.sub.1 to the transistor Q.sub.2 and from the transistor Q.sub.3 to the transistor Q.sub.4. In order to prevent the power loss in the standby state, however, a problem arises that a heavy and expensive electromagnetic relay RY of a two-circuit one-contact type must be selected.
Further, since the resonant current I' flowing through the secondary winding N.sub.1 ' of the saturable power regulation transformer PRT becomes a high-frequency current of 20A.sub.P-P when the main load current is 45W, an electromagnetic relay RY of which the contact has a large current capacity is required.